1. Field of the Invention
The present invention relates to an acoustic apparatus in which a vibrator is arranged in a Helmholtz's resonator having a resonance port comprising an open duct, and is driven to radiate a resonant acoustic wave and, more particularly, to an acoustic apparatus in which the vibrator is driven to cancel an air reaction from the resonator side when the Helmholtz resonator is driven, thereby allowing lower base sound reproduction using a smaller cabinet.
2. Description of the Prior Art
As an acoustic apparatus solely utilizing a Helmholtz's resonance, a phase-inversion (bass-reflex) speaker system is known. FIGS. 13A and 13B are respectively a perspective view and a sectional view showing an arrangement of the bass-reflex speaker system. In the speaker system shown in FIGS. 13A and 13B, a hole is formed in the front surface of a cabinet 1, a vibrator (speaker unit) 4 consisting of a diaphragm 2 and a dynamic speaker 3 is mounted in the hole, and an open duct port (resonance port) 8 having a sound path 7 whose opening 6 is open to an external portion is formed therebelow. In the bass-reflex speaker system according to the conventional basic design, a resonance frequency (antiresonance frequency) f.sub.OP defined by an air spring of the cabinet 1 and an air mass in the sound path 7 is set to be lower than a lowest resonance frequency f.sub.OC of the vibrator 4 when the vibrator is assembled in the bass-reflex enclosure, and in some cases, lower than a lowest resonance frequency f.sub.O inherent to the vibrator. At a frequency higher than the antiresonance frequency f.sub.OP, the phase of sound pressure from the rear surface of the diaphragm 2 is inverted at the sound path 7. Consequently, in front of the cabinet 1, a sound directly radiated from the front surface of the diaphragm 2 is in phase with a sound from the opening 6, and these sounds are in-phase added to each other, thus increasing the sound pressure. As a result of the in-phase addition, the lowest resonance frequency of the system is expanded to the antiresonance frequency f.sub.OP of the resonator, and according to an optimally designed bass-reflex speaker system, the frequency characteristics of the output sound pressure can be expanded to the resonance frequencies f.sub.OC and f.sub. O of the vibrator 4 or less. As indicated by an alternate long and two short dashed curve in FIG. 14, a uniform reproduction range can be widened as compared to an infinite plane baffle or closed baffle.
In the bass-reflex speaker system, a frequency of an antiresonance (resonance) is as low as 50 to 100 Hz. Thus, a wavelength of a sound is long, and an interference due to a difference in distance between the front surface of the diaphragm 2 and the opening 6, i.e., an interference between two sound generation sources, is not conspicuous. In particular, in view of the dimensions of a normal speaker cabinet, the speaker system can hardly have a propagation difference (an odd-number multiple of a half wavelength) large enough to achieve opposite phase cancellation. However, in the conventional bass-reflex speaker system, a phase relationship has a primary importance in principle, and in order to accurately perform in-phase addition, the two sound generation sources, i.e., the diaphragm 2 and the opening 6 should be arranged parallel on a single plane, in particular, on the front surface of the cabinet facing to reproduce sound radiation direction at the same level. If the length of the resonance port 8 is increased in order to further increase the bass sound reproduction range and the like and the propagation difference is increased, a problem of an interference is inevitably posed.
In the conventional speaker system, a large cabinet must be basically used to satisfactorily reproduce a bass sound, and no means can eliminate this drawback. Although the bass-reflex speaker system can reproduce a lower bass range than a closed speaker system if the cabinet volume remains the same, a large cabinet to some extent is normally used. The antiresonance frequency f.sub.OP is given by: EQU f.sub.OP =c(S/lV).sup.1/2 /2.pi. . . . (1)
where c is the sonic speed, S is the sectional area of the resonance port 8 (area of the opening 6), l is the length of the port 8, and V is the volume of the cabinet 1.
Therefore, since the cabinet volume V is large and hence S/l can be large accordingly, the resonance port 8 for realizing a low antiresonance frequency f.sub.OP can be short. For this reason, there is no problem when the diaphragm 2 and the opening 6 are arranged parallel on the front surface of the cabinet 1 at the same level. Thus, the port 8 has never become too long to be assembled in the cabinet. For example, the port length has never exceeded the depth of the cabinet 1.
A resonance frequency f.sub.OP of a Helmholtz's resonator constituted by the cabinet 1 and the resonance port 8 may be extremely decreased regardless of the basic concept of a bass-reflex speaker system. In this case, in a drive method using a conventional power amplifier, the Q value of the speaker unit is increased and the Q value of the resonator is decreased due to mutual dependency of the speaker unit and the resonator. Thus, a sufficient bass sound resonance radiation power of the resonator cannot be assured. Such a change in Q value is conspicuous as the diameter of the resonance port 8 is decreased or as its length is increased or when the port 8 is bent or the opening portion at the trailing end of the port 8 approaches the inner surface of the cabinet by a distance equal to or smaller than the inner diameter of the port. Therefore, in the conventional bass-reflex speaker system, it is considered to be impossible or very difficult to render the cabinet compact and to expand the bass sound reproduction range so that the resonance port 8 must be lengthened and bent or project from the front surface of the cabinet.